feat(*): export a version

The version is currently no used but can be used in
the future to check for compatibility when ViewportProviders
with different versions are rendered within the same tree.
Therefore it is required to introduce it now.

Issue: #3

Author: garthenweb

lib/ObserveViewport.tsx

@@ -36,6 +36,7 @@ interface IContext {
   ) => void;
   removeViewportChangeListener: (handler: TViewportChangeHandler) => void;
   hasRootProviderAsParent: boolean;
+  version: string;
 }
 
 export default class ObserveViewport extends React.Component<IProps, IState> {

lib/ViewportProvider.tsx

@@ -7,6 +7,7 @@ import {
   IViewportCollectorUpdateOptions,
 } from './types';
 import ViewportCollector from './ViewportCollector';
+import { VERSION } from './index';
 
 interface IListener extends IViewportChangeOptions {
   handler: TViewportChangeHandler;
@@ -19,6 +20,7 @@ const ViewportContext = React.createContext({
     options: IViewportChangeOptions,
   ) => {},
   hasRootProviderAsParent: false,
+  version: VERSION,
 });
 
 export const Consumer = ViewportContext.Consumer;
@@ -67,6 +69,7 @@ export default class ViewportProvider extends React.PureComponent {
         addViewportChangeListener: this.addViewportChangeListener,
         removeViewportChangeListener: this.removeViewportChangeListener,
         hasRootProviderAsParent: true,
+        version: VERSION,
       };
       return (
         <React.Fragment>

lib/index.ts

@@ -5,3 +5,5 @@ export {
 } from './ObserveBoundingClientRect';
 export { default as ObserveViewport } from './ObserveViewport';
 export { IRect, IScroll, IDimensions } from './types';
+
+export const VERSION = '__VERSION__';